This invention relates to the geological study of subsoils notably for the location and exploitation of mineral deposits.
Mineral and petroleum prospecting is based upon the geological study and observation of formations of the earth's crust. Correlations have long been established between geological phenomena and the formation of mineral deposits which are sufficiently dense to make their exploitation economically profitable.
In this endeavor, the study of the facies of the rocks encountered takes on particular importance. By facies, notably of a sedimentary rock, is meant a set of characteristics and properties of a rock which result from the physical, chemical and biological conditions involved in the formation of the sediment and which have given it its distinctive appearance with respect to other sediments. This set of characteristics provides information on the origin of the deposits, their distribution channels and the environment within which they were produced. For example, sedimentary deposits can be classified according to their location (continental, shoreline or marine), according to their origin (fluviatile, lacustrine, eolian) and according to the environment within which they occurred (estuaries, deltas, marshes, etc.). This information in turn makes it possible to detect, for example, zones in which the probability of hydrocarbon accumulation is high.
There are various sources of information on the facies of formations. It may be provided by surface or subsoil observations, and notably by the study of core samples taken from rock studies, for example during the drilling of a borehole for an oil well.
The geological characteristics used for recognizing a facie include, in addition to the fossil fauna and flora:
The mineralogy, i.e. the mineral composition of the rock; silicate, carbonate, evaporite, etc.;
The texture: grain size, sorting and morphology, degree of compaction, of cementation, etc.; these parameters can be of decisive importance as concerns the permeability of rocks also exhibiting porosity values and other similar ones; they are related to the microscopic appearance of the rocks;
The structure: thickness of beds, their alternation, presence of stones, lenses, fractures, degree of parallelism of laminations, thickness of strata, etc.: all of which are parameters related to the macroscopic appearance of the rocks.
The petrophysical and petrographic characteristics of a rock, excluding the paleontological data, constitute the litho-facies of the rock. This consequently includes the descriptive characteristics of the rock independent of the genetics of formation and notably of deposition.
Other types of information coming from the subsoil can be used by the geologist for the investigation of facies. Such information can be provided by drill cuttings sent up to the surface from the bottom of a well by means of a fluid (generally mud) injected near the drilling tool.
It has already been noted that certain measurements of the physical characteristics of the formations traversed by a borehole made it possible to obtain valuable information for the interpretation of the facies.
Such measurements for determining the physical characteristics of the geological formations traversed by boreholes are presently carried out on a very large scale, notably in oil wells. They are carried out by means of sondes moved in the borehole, and the signals transmitted by the sonde give a recording (log) as a function of depth. They may involve highly varied characteristics resulting either from natural phenomena, such as the spontaneous potential or the natural emission of gamma rays, or from a prior stimulation of the formation by the sonde by the emission of electric current or acoustic waves, electromagnetic waves, nuclear particles, etc.
In the case of petroleum prospecting, the logs are useful in determining accurately the hydrocarbon-bearing strata and at investigating, in addition to the nature and quantity of such strata, the possibility of extracting the hydrocarbons from the rocks in which they are contained.
A substantial part of the log interpretation efforts up to the present time has tended toward the evaluation of the porosity of the reservoir rocks or matrices and their permeability, as well as the fraction of the pore volume occupied by these hydrocarbons. These so-called formation evaluation techniques generally also bring out other parameters such as the average matrix rock grain density and clay content.
These interpretation studes have also demonstrated and used correlations between the measurements furnished by logging tools and certain compositional characteristics of the rocks traversed by boreholes. For example, it is common to plot certain information on the readily identifiable lithology of the formations encountered as a function of borehole depth, a prime example being the proportion of limestone and dolomite of a rock at a given level of the well.
More recent studies have shown that very clear relations could in fact exist between the appearance or the evolution of certain characteristics in logging measurements and certain parameters of the litho-facies.
This raised the question as to whether it would not be possible to establish a correspondence between the different facies or litho-facies encountered within a formation interval and all the logging data obtainable in this interval so as to establish an "Electro-facies" or a "para-facies" constituting an image of the facies or of the litho-facies of the rock as seen through the logs.
This idea is based upon the notion according to which each log represents a response spectrum characteristic of the facies of the different zones along which it has been established, and that all these logs represent a respective "signature" of these facies or litho-facies.
It has however not been possible up to the present time to develop a method which, in a relatively constant, reliable and systematic manner, makes it possible to establish, from only the logging measurements made in a borehole over a given depth interval, a recording or a respresentation notably in the form of a graph which furnishes, as a function of depth, an image of the succession of the litho-facies present within this interval.
More specifically, it would be desirable to be able, on the basis of log measurements over a given depth interval in a borehole, to show within this borehole a set of sections or zones capable of being classified in correspondence (at least approximate) with the different facies or litho-facies present in the interval, so that all the zones corresponding to a similar facies or litho-facies belong to the same class.